The problem with this line of thought is that ones codex is usually properly balanced within itself to adequately make up for whatever weaknesses it has already.

Um....Have you read the non-marine codex because that is clearly not true of all codexs?

Edit: That said lets assume I agree for a moment. Just because the Tau are (in theory) shooty enough to make up a lack of CC that does not mean they will be unable to benefit from having some heavily armored CC friendly troops which can help them hold objectives they previously would have trouble even contesting.

Not when you get down to the nitty gritty min-maxing, no. Most of those other books were written in 4th, went through the changes in fifth and now the changes in 6th, obviously there is going to be at least a bit of disconnect between cost effectiveness then and cost effectiveness now. That's not the fault of the Codex though, that hasn't changed. It's the fault of GW not updating the books in a timely manner.

I have read through nearly every codex out, and clearly the situation isn't as bad as you think it is because at the end of the day people are still playing those armies.

I know what its like to be left behind. I play Tau, which only gets to participate in about half of the game (No assault, no psyker, no mc, no walker, no bikes). But we can still play, and we can still win.

yeah, i can agree with that. and it is nice to see gw using the "amendment" section of the faq's for 6th. in 5th, they didn't do any types of minor fixes for balance, they just left them as they were. hopefully the "new" gw will update the codices in a nice order rather than what they feel like so no one gets left behind.

I have read through nearly every codex out, and clearly the situation isn't as bad as you think it is because at the end of the day people are still playing those armies.

I must respectfully disagree. You are just not seeing that reflected at tournaments which in terms of this particular conversation are the real metric here. 0% SoB and Tyranids at many tournaments and pitiable numbers of other Xenos speaks very loudly. Some codexs are at fault over costing units and leaving clear deficiencies (not just strong and weak points) in some armies even within the proper edition.

Yes these armies are still played, but given the expense and investment of time one puts into an army it isn't a casual thing to just pick up a new army and play which makes people far more loyal than they would be otherwise. Additionally some people like a challenge and pick the weaker armies deliberately. This isn't uncommon at all. Most people in magic have several "pet decks" which they play for challenge and one "win deck" which they drag out when they want to get serious and win rather then just have a fun game. Most people in 40k seems to do the same, have a fun army and a win army. Any army can win I agree, but only a few armies can win consistently against a diverse set of opponents. And the very slow to change meta game means those top armies stay on top for a good while.

Tyranids for example is a terrible codex, but it is still fun. I really enjoy the parasite of motrex and biovores but almost no one would use those units in a game someone meant to win. A unit or a whole codex can still find a way to be enjoyable and the army will still be played even if it is a terrible codex. And yes, terrible is probably overstating things but I think it is very hard to deny the evidence that in the metagame prior to 6th release their were very clear winners and losers

I think it's still far too early into this edition to be able to speak about how balanced it is.

I'll still concede and say that my comment was a bit wishful.

No harm in a little wishful thinking, I certainly wish I could agree with you 100% and the reality was each codex was really that good. Also I certainly agree that 6th is still shaping up and you make a fair point about the better designed armies which are balanced within the codex actually could be weakened by allies. Is just sadly I think not all codex are so well balanced that dividing their strength between two armies would be a negative.

Well, sharing strength is more like averaging it out though. A main army of 5 strength is allied to something of 7 strength, the average being 6. On the whole, this will weaken the 7 strength army.

This is what makes the allies rules more or less useless, because if you're using the allies to cover your weaknesses, you might as well just be playing that allied army because you already have the models, you already have the book, and they don't have that weakness which you want covered.

Well, sharing strength is more like averaging it out though. A main army of 5 strength is allied to something of 7 strength, the average being 6. On the whole, this will weaken the 7 strength army.

This is what makes the allies rules more or less useless, because if you're using the allies to cover your weaknesses, you might as well just be playing that allied army because you already have the models, you already have the book, and they don't have that weakness which you want covered.

Not necessarily. Sometimes a thing is more then the sum of its parts. I think the deathcult, a unit which runs contrary to all the rest of the Sisters units, shows that.

The deathcult ends up being extremely good because it fills a gap in the sisters army. The problem is for practical purposes you can only take one. If instead of getting a death cult we spent those points an dark angels picking terminators now we can have up to three groups of CC specialists with better protection and some shooting ability without giving up the sister's strength such as our powerful Heavy Support slots in the form of exorsists and our powerful HQ choice of Celestine. Further with the terminators helping to protect our Battle Sisters from CC they can actually be more effective now able to move into rapid fire range without worrking about being swept aside into CC.

If I were to just play dark angels I could not do that. I couldn't take celestine and I couldn't benefit from exorsists and that would result in a very different play style with very different strengths and weaknesses. Might it be better to go pure DA? I can't say, but I can say that dark angels play very differently from a SoB+DA alliance army does.

The final point I would leave on is that Sisters of Battle are and have always been designed to be an ally or have an ally to fill out their ranks. Their roster has never been a complete one so, even your assertion about allies is true for every other army out there the Sisters are still an exception, an army meant to act with another army to make a stronger whole, by design. If we assume the other codex's create a balanced fighting force because they are designed too, we must then assume the sisters make a formidable alliance because that is what they are designed to do.